[Telephone] exchanges consisted of one to several hundred plug boards staffed by telephone operators. Each operator sat in front of a vertical panel containing banks of jacks, each of which was the local termination of a subscriber's telephone line.

When a calling party clicked the reciever, a signal lamp near the jack would light. The operator would plug one of the cords (the "answering cord") into the subscriber's jack and switch her headset into the circuit to ask, "number please?" Depending upon the answer, the operator might plug the other cord of the pair (the "ringing cord") into the called party's local jack and start the ringing cycle, or plug into a trunk circuit to start what might be a long distance call handled by subsequent operators in another bank of boards or in another building miles away.

In 1918 the average time to complete a long-distance call was 15 minutes. In the ringdown method, the originating operator called another intermediate operator who would call the called subscriber, or passed it on to another intermediate operator.In 1918 the average time to complete a long-distance call was 15 minutes. In the ringdown This chain of intermediate operators could complete the call only if intermediate trunk lines were available between all the centers at the same time. In 1943 when military calls had priority, a cross-country US call might take as long as 2 hours to request and schedule in cities that used manual switchboards for toll calls. -- Wikipedia